The neural basis of variation in human intelligence is not well delineated. Numerous studies relating measures of brain size such as brain weight, head circumference, CT or MRI brain volume to different intelligence test measures, with variously defined samples of subjects have yielded inconsistent findings with correlations from approximately 0 to 0.6, with most correlations approximately 0.3 or 0.4. The study of intelligence in relation to postmortem cerebral volume is not available to date. We report the results of such a study on 100 cases (58 women and 42 men) having prospectively obtained Full Scale Wechsler Adult Intelligence Scale scores. Ability correlated with cerebral volume, but the relationship depended on the realm of intelligence studied, as well as the sex and hemispheric functional lateralization of the subject. General verbal ability was positively correlated with cerebral volume and each hemisphere's volume in women and in right-handed men accounting for 36% of the variation in verbal intelligence. There was no evidence of such a relationship in non-right-handed men, indicating that at least for verbal intelligence, functional asymmetry may be a relevant factor in structure-function relationships in men, but not in women. In women, general visuospatial ability was also positively correlated with cerebral volume, but less strongly, accounting for approximately 10% of the variance. In men, there was a non-significant trend of a negative correlation between visuospatial ability and cerebral volume, suggesting that the neural substrate of visuospatial ability may differ between the sexes. Analyses of additional research subjects used as test cases provided support for our regression models. In men, visuospatial ability and cerebral volume were strongly linked via the factor of chronological age, suggesting that the well-documented decline in visuospatial intelligence with age is related, at least in right-handed men, to the decrease in cerebral volume with age. We found that cerebral volume decreased only minimally with age in women. This leaves unknown the neural substrate underlying the visuospatial decline with age in women. Body height was found to account for 1-4% of the variation in cerebral volume within each sex, leaving the basis of the well-documented sex difference in cerebral volume unaccounted for. With finer testing instruments of specific cognitive abilities and measures of their associated brain regions, it is likely that stronger structure-function relationships will be observed. Our results point to the need for responsibility in the consideration of the possible use of brain images as intelligence tests.

The anatomy of the sylvian fissure in the human brain was studied to develop reliable criteria for anatomical landmarks of the posterior part of the fissure for use in its definition and measurement; to quantify right-left asymmetries in segments of the sylvian fissure; to assess whether any anatomical features are associated with hand preference (selected as one index of hemispheric functional asymmetry); and whether structure-function relationships are similar in men and women. A sample of 67 brain specimens (24 men and 43 women, mean age = 53 years) was studied postmortem (with the aid of dissection) from people who had been tested before death for detailed hand preference. Sylvian fissure anatomy in the human brain is very variable and no agreement exists as to the point of its posterior termination. The posterior ascending ramus, originating at the posterior bifurcation of the fissure, was found to be the continuation of the main limb of the sylvian fissure. Three segments of the sylvian fissure were defined and measured: anterior, horizontal, and vertical. The anterior segment showed no asymmetry; the horizontal segment was twice as large on the left side as on the right; and the vertical segment twice as large on the right. The two asymmetries counterbalanced each other, and overall asymmetry in the posterior region (horizontal plus vertical) was minimal. The basic asymmetry is in the position at which the fissure turns up, resulting in the different extent and position of the surrounding right and left parietal and temporal gyri and associated cytoarchitectonic regions. The possible embryological course of the asymmetry is discussed. Handedness correlated with anatomy of the sylvian fissure in men. In contrast to general expectation, hand preference was associated with a bilateral feature of morphology, and not with less asymmetry in non-right-handers. Men having consistent-right-hand preference had longer horizontal segments in both hemispheres compared to men not having consistent-right-hand preference. The direction and magnitude of asymmetry did not differ between the two male hand-preference groups. Since hand preference is an index of other motor and perceptual functions which are asymmetrically represented in the two hemispheres in gyri surrounding the sylvian fissure, it is suggested that anatomy of the sylvian fissure is related to functional asymmetries in men. A sex difference in structure-function relationship was observed. No association was found between hand preference and sylvian fissure anatomy in women.(ABSTRACT TRUNCATED AT 400 WORDS)

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The results of several studies have shown that homosexual men have an increased prevalence of non-right-handedness and atypical patterns of hemispheric functional asymmetry. Non-right-handedness in men has been associated with increased size of the corpus callosum (CC), particularly of the isthmus, which is the posterior region of the callosal body connecting parietotemporal cortical regions. We hypothesized that isthmal area would be greater in homosexual men, even among right handers. Twelve homosexual and ten heterosexual healthy young men, all consistently right-handed, underwent a research-designed magnetic resonance imaging scan. We found that the isthmal area was larger in the homosexual group, adding to the body of findings of structural brain differences between homosexual and heterosexual men. This result suggests that right-handed homosexual men have less marked functional asymmetry compared to right-handed heterosexual men. The results also indicate that callosal anatomy and laterality for motoric functions are dissociated in homosexual men. A logistic regression analysis to predict sexual orientation category correctly classified 21 of the 22 men (96% correct classification) based on area of the callosal isthmus, a left-hand performance measure, water level test score, and a measure of abstraction ability. Our findings indicate that neuroanatomical structure and cognition are associated with sexual orientation in men and support the hypothesis of a neurobiological basis in the origin of sexual orientation.